Lead-free solder ball

ABSTRACT

Lead-free solder balls having a smooth surface and no shrinkage cavities or wrinkles are made of a lead-free solder which comprises, by atomic percent, 3%-6% of Ag, 1%-4% of Cu, 0.01%-2% of at least one element of the iron group and preferably Co, optionally 0.04%-4% of P, and a balance of Sn.

TECHNICAL FIELD

[0001] The present invention relates to a lead-free solder ball suitablefor use in soldering of electronic components.

BACKGROUND ART

[0002] Nowadays, as electronic products are required to bemultifunctional and have a reduced size, electronic components, whichform the core of electronic products, are also required to bemultifunctional and have a reduced size.

[0003] In conventional electronic components, a semiconductor chip ismounted on a lead frame made of a metal such as copper or Alloy 42 (42Ni—Fe alloy), and the chip is electrically connected to the lead frameby wire bonding with gold wires before being sealed with ceramic orplastic. However, with such an electronic component using a lead frame,there is a limit on the extent to which the size of the electroniccomponent can be reduced since the lead frame occupies its own space ona printed circuit board. The use of a lead frame also puts a limit onthe speed of operation, since the lead frame increases the length ofelectrical connections on which the speed of operation depends.

[0004] In view of these problems of electronic components using a leadframe, another type of electronic component called a BGA (ball gridarray) package was developed. A BGA package employs solder bumps formedfrom solder balls in order to connect and secure the package to aprinted circuit board. Therefore, the length of electrical connection isshorter than for electronic components using lead frames, thereby makingit possible to increase the speed of operation. The space occupied by alead frame is no longer necessary, so a BGA package also makes itpossible to save space. Recently, more compact BGA packages called FBGA(fine ball grid array) packages or CSP's (chip size packages) havingnearly the same size as the chips packaged therein and having a finerelectrode pitch have been produced. BGA packages, including CSP's, arenow widely used, and solder ball-mounting technology is becomingprevalent in the mounting of electronic components.

[0005] A BGA package has a substrate on which a semiconductor chip ismounted. The substrate has solder bumps on its back surface which areformed from solder balls arranged in a grid-like array. In a typicalmethod for forming the solder bumps, a solder ball feeder equipped witha suction plate is used. The suction plate has holes arranged in thesame grid-like array as the solder bumps to be formed. Solder balls arepositioned on a substrate by pulling a solder ball into each hole of thefeeder by suction applied through the holes, and after positioning thefeeder above a substrate, releasing the suction to place or mount eachball onto the substrate. The solder balls are temporarily kept inposition on the substrate by the stickiness of a soldering flux, whichhas previously been applied to the surface of the substrate on whichsolder bumps are to be formed. The substrate having the solder ballsmounted thereon is then heated in a reflow furnace to melt the solderballs and form them into solder bumps secured to the substrate. Thesubstrate having solder bumps thus formed is normally inspected by anoptical inspection machine in order to check if all the solder bumpsrequired to make the desired grid-like array are properly formed.

[0006] Solder balls which are used in the production of BGA packages,including CSP's, have a spherical shape with a diameter in the range of0.05-2.0 mm. It is desired for solder balls to have good sphericity anda smooth surface. If the sphericity of a solder ball is not good or ifits surface has significant surface irregularities or defects such asshrinkage cavities or wrinkles, problems may occur during placement ofthe solder ball on a substrate, such as caused by failure of the feederto pick up the solder ball due to a loss of suction or a failure of thefeeder to release the solder ball due to biting of the ball into one ofthe holes in the feeder.

[0007] It is also desired for solder balls to form solder bumps having asmooth surface with a uniform gloss when heated in a reflow furnace. Theoptical inspection of solder bumps formed on a substrate is conducted byfocusing normally on solder bumps having a glossy surface or in somecases on solder balls having a non-glossy surface, so the formation ofsolder bumps, some of which have a glossy surface and others of whichhave a non-glossy surface, makes it difficult to adjust the focus in theoptical inspection of the solder bumps and results in a failure toidentify some of the bumps.

[0008] The solder that has been used most widely in soldering is anSn—Pb alloy. Sn—Pb solder has been used since ancient times and has theadvantages of a low melting point and good solderability. In addition,an Sn—Pb alloy having an Sn content of 63 wt % or 75 at %, which is arepresentative composition for an Sn—Pb solder, has the excellentproperties that it forms a soldered joint having a smooth surface withgood gloss.

[0009] Solder balls made of an Sn—Pb alloy have a smooth surface, sothey can be smoothly mounted onto a substrate by use of theabove-described solder ball feeder. In addition, after heating in areflow furnace, they form solder bumps having a smooth and glossysurface which does not interfere with optical inspection of the solderbumps.

[0010] Recently, the use of an Sn—Pb solder has been disfavored due tothe toxic nature of Pb. When waste electronic product such as computersare disposed of, they are normally disassembled to remove plastic andmetallic parts for recycling. Printed circuit boards on which electroniccomponents are mounted are not adapted for recycling since plastic andmetallic portions are combined therein, so the printed circuit boardsare removed from waste electronic products, shredded, and buried in theground. When rain which has been acidified due to air pollution contactsshredded printed circuit boards buried in the ground, the lead (Pb) inthe Sn—Pb solder may be dissolved out and contaminate underground water.If a human or animal continues to drink lead-containing water for manyyears, there is the possibility of lead accumulating in its body andcausing lead poisoning.

[0011] Accordingly, it is now highly recommended, from an environmentalstandpoint, to use a “lead-free” solder, which is completely free fromlead, in soldering of electronic components.

[0012] Lead-free solders which are considered promising at present areSn—Ag solders and particularly Sn—Ag—Cu solders in view of their ease ofhandling. However, the wettability of these lead-free solders isgenerally lower than that of Sn—Pb solders. For example, in a spreadingtest, an Sn—Ag—Cu lead-free solder shows a spreading factor ofapproximately 80% that of an Sn—Pb solder.

[0013] When solder balls are made from such an Sn—Ag—Cu lead-freesolder, the surfaces of the resulting solder balls have shrinkagecavities and wrinkles, thereby increasing the occurrence of theabove-described problems during mounting of solder balls on a substrateusing the above-described solder ball feeder. In addition, after thesolder balls are heated in a reflow furnace to form solder bumps, thesurfaces of the resulting solder bumps tend to be glossy for some bumpsand non-glossy for other bumps. The formation of glossy bumpsinterspersed with non-glossy bumps makes optical inspection of thesolder bumps difficult and increases the rate of misidentification.These disadvantages have been an impediment to the use of lead-freesolders as a material for solder balls in place of conventional Sn—Pbsolders.

DISCLOSURE OF THE INVENTION

[0014] The present invention provides a solder ball of anSn—Cu—Ag-based, lead-free solder which has a smooth surface havinglittle or no shrinkage cavities or wrinkles.

[0015] More particularly, in one aspect, the present invention providesa solder ball made of a lead-free solder which comprises, by atomicpercent, 3%-6% of Ag, 1%-4% of Cu, 0.01%-2% of at least one element ofthe iron group and preferably Co, optionally 0.04%-4% of P, and abalance of Sn.

[0016] In another aspect, the present invention also provides asubstrate for a BGA package (including CSP) which have solder bumpsformed from the above-described solder balls.

[0017] The present invention also relates to a method of forming solderbumps on a substrate comprising placing the above-described solder ballson the substrate followed by heating the substrate to melt the solderballs and form them into solder bumps secured to the substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018]FIG. 1 is an electron photomicrograph of solder balls of anSn—Cu—Ag—Co lead-free solder according to the present invention.

[0019]FIG. 2 is an electron photomicrograph of solder balls of anSn—Cu—Ag lead-free solder.

[0020]FIG. 3 is an electron photomicrograph of solder balls of aconventional Sn—Pb solder.

DETAILED DESCRIPTION OF THE INVENTION

[0021] Spherical solder balls can be produced either by remelting solidmasses of solder of a certain size followed by cooling, or by formingdroplets of a certain size from molten solder followed by cooling. Ineither method, molten solder of a certain size is cooled and changedinto a solid. In the course of solidification, some of the elements ofthe solder initially crystallize out in the liquid mass, and theresulting crystals, which serve as nuclei for crystal growth, graduallygrow until the entire mass becomes solid. This crystal growth maysometimes occur unidirectionally, i.e., in a certain direction, therebyforming dendrites.

[0022] When molten solder is cooled so as to solidify, a conventionalSn—Pb solder, and particularly a eutectic Sn—Pb solder finishessolidifying in a short time. In contrast, an Sn—Cu—Ag solder takes alonger time to finish solidifying, as suggested by its DSC (differentialscanning calorimetry) curve in which the solidifying peak is wider thanthat of an Sn—Pb solder. In addition, because of the actual coolingspeed during solidification which is not slow enough to result inequilibrium, the alloy composition at the final solidification stagedeviates from the eutectic composition. With an Sn—Cu—Ag solder, thisdeviation becomes greater than with an Sn—Pb solder, and the degree ofsupercooling is also greater. As a result, during transition from aliquid to a solid phase, solid solution tends to form excessively,thereby causing the formation of dendrites or coarse crystals, whichwhen grown, lead to the formation of surface detects such as shrinkagecavities and wrinkles and significant surface irregularities.

[0023] During solidification of molten solder, if a large number ofnuclei which initially crystallize out are present in the molten solder,it becomes difficult to form a solid solution excessively, which in turnmakes it difficult to form dendrites or coarse crystals.

[0024] According to the present invention, at least one of the irongroup elements (Fe, Ni, and Co) and preferably Co is added to anSn—Cu—Ag solder in an amount of 0.01-2 atomic percent (at %). Each ofthese elements has a melting point which is much higher than those ofSn, Cu, and Ag. Therefore, during solidification of molten solder, theseelements initially crystallize out to form a large number of nuclei forcrystal growth. It has been found that addition of at least one elementof the iron group and particularly addition of Co is effective forproducing solder balls having a smooth surface in which the formation ofshrinkage cavities and wrinkles is prevented, thereby making it possibleto produce solder balls which can be smoothly mounted on a substrateusing a solder ball feeder of the above-described type. The solder ballscan form solder bumps having a uniform glossy surface, which facilitatesoptical inspection and minimizes the occurrence of misidentification.

[0025] These effects are not appreciable if the total content of theiron group elements is less than 0.01 at %. If this content is more than2 at %, the iron group elements become segregating on the surface of thesolder balls without dissolving into the inside thereof, therebydeteriorating the wettability of the solder balls when melted andadversely affecting the surface smoothness and gloss thereof.Preferably, the iron group element is cobalt (Co). The total content ofthe iron group elements is preferably in the range of 0.02-0.5 at %.

[0026] The addition of at least one element of the iron group to anSn—Ag—Cu lead-free solder is disclosed in U.S. Pat. No. 6,179,935 and6,231,691 and JP P11-216591A (1999). However, there is no disclosuretherein regarding the production or use of solder balls for bumpformation or the surface appearance of the solder. The ball formreferred to in column 12 of U.S. Pat. No. 6,179,935 is the shape ofsolder powder for use in a cream solder.

[0027] The Ag and Cu contents of the alloy composition in a lead-freesolder ball according to the present invention are 3-6 at % and 1-3 at%, respectively.

[0028] When present in an amount of at least 3 at %, Ag serves to lowerthe melting point of the solder and improve the wettability and strengththereof. However, when the Ag content increases so as to exceed 6 at %,it adversely affects both the melting temperature and wettability of thesolder. Preferably, the Ag content is in the range of 3-5 at %. In orderto lower the melting point of the solder, it is also preferable that theAg content be selected such that the atomic ratio of Ag to Sn isapproximately 3:70.

[0029] When present in an amount of at least 1 at %, Cu serves toimprove the strength of the solder. The presence of a small amount of Cualso provides the solder with improved wettability. However, like Ag,the presence of an excessive amount of Cu, which is greater than 4 at %for Cu, causes a rise in the melting temperature of the solder anddeteriorates its wettability. Preferably, the Cu content is in the rangeof 1-3 at %. More preferably, the Cu content is selected such that theatomic ratio of Cu to Sn is approximately 1:70.

[0030] Although the addition of at least one iron group element andpreferably Co is effective for preventing the formation of dendritesduring solidification and producing solder balls having a smooth surfacewith no shrinkage cavities or wrinkles, the addition can possiblyadversely affect the wettability of the solder balls. In order toeliminate this possibility, phosphorus (P) may be added in a smallamount. Thus, the addition of P ensures that the solder can exhibit goodwettability even though it contains at least one iron group element.When added, the content of P is in the range of 0.04-4 at %.

[0031] In a solder ball according to the present invention, Sn is theremainder of the solder composition. In general, the Sn content is inthe range of 86-96 at %.

[0032] In addition, the solder composition may contain unavoidableimpurities such as Pb, Sb, and Bi in a total amount of at most 0.2 at %.

[0033] There is no limitation on the diameter of a solder ball accordingto the present invention as long as the solder ball is suitable for usein the formation of solder bumps on a substrate for a BGA package orCSP. In general, the diameter is in the range of from 0.05 mm to 1.0 mm.

[0034] Solder balls according to the present invention can be producedby a method which comprises forming masses of molten solder of theabove-described composition having almost equal volumes and solidifyingthe masses to form balls having almost equal diameters.

[0035] Examples of such a ball forming method include an oil bath methodas disclosed in U.S. Pat. No. 5,653,783 and JP P07-300606A (1995) and adirect method as disclosed in U.S. Pat. No. 5,445,666, although othermethods may be employed.

[0036] In the oil bath method, a wire of a solder having a predeterminedcomposition is prepared and cut into portions having a given length. Thewire portions are separately dropped into an oil bath having a verticaltemperature gradient in which the temperature in an upper portion of thebath is higher than in a lower portion, whereby the portions are allowedto melt in the upper portion and then solidify while falling in the oilbath.

[0037] In the direct method, a molten solder having a predeterminedcomposition is prepared. The molten solder is dripped or allowed to fallin droplets of a given size through an orifice or nozzle, and thensolidified while falling in a chamber.

[0038] In both of these methods, the resulting solder balls have aspherical shape due to the action of the surface tension of the moltensolder.

[0039] The following examples are presented to further illustrate thepresent invention. These examples are to be considered in all respectsas illustrative and not restrictive.

EXAMPLES

[0040] Solder balls having a diameter of 0.5 mm were produced from eachof various solders having the compositions shown in Table 1 by aconventional oil bath method, and they were used to form solder bumps ona substrate for a BGA package by mounting the solder balls on thesubstrate using a solder ball feeder having holes for grasping solderballs by suction and then heating the substrate in a reflow furnace at atemperature sufficient to form solder bumps.

[0041] The reliability of mounting solder balls was evaluated by thepercentage of solder balls with respect to which problems occurredduring mounting of solder balls, i.e., the percentage of solder ballswhich were not grasped by the solder ball feeder by suction or whichwere not released from the feeder due to biting into the holes of thefeeder.

[0042] In addition, the solder bumps formed on the substrate werechecked by an optical inspection machine developed for checking solderbumps to determine the percent occurrence of misidentification of solderbumps in this inspection (the percentage of solder bumps which were notidentified by the inspection machine).

[0043] The results obtained with each solder are also shown in Table 1together with the surface appearance of the solder balls when observedunder a scanning electron microscope (SEM) and the spreading factordetermined in a conventional spreading test on a copper plate.

[0044] Electron photomicrographs of solder balls of Run No. 1, No. 6,and No. 7 in Table 1 are shown in FIGS. 1, 2, and 3, respectively. TABLE1 Run Solder composition Ball % Mounting % Misidenti- Spreading No.(atomic %) surface problems fication¹ factor (%) Remarks 1Sn-3.8Ag-1.3Cu-0.02Co smooth 0.01 0.04 82 Invent.³ 2 Sn-3Ag-1.0Cu-0.02Cosmooth 0.01 0.04 80 Invent. 3 Sn-5Ag-3.5Cu-0.02Co- smooth 0.01 0.04 78Invent. 3.4P 4 Sn-3.8Ag-1.3Cu-0.2Co smooth 0.01 0.04 82 Invent. 5Sn-3.8Ag-1.3Cu-1.0Co- smooth 0.03 0.06 82 Invent. 3.4P 6 Sn-3.8Ag-1.3Cuirregular² 0.08 1.2 82 Compar.⁴ 7 Sn-25Pb smooth 0.01 0.03 92 Compar.

[0045] As can be seen from FIG. 2 and in Table 1, solder balls made of aconventional Sn—Cu—Ag lead-free solder in Run No. 6 showed significantsurface irregularities and had shrinkage cavities on their surface whenobserved under a SEM, thereby resulting in significantly increasedpercentages of occurrence of mounting problems for solder balls andmisidentification in optical inspection of solder bumps.

[0046] In contrast, as can be seen from FIG. 1 which shows solder ballsmade of a lead-free solder of Run No. 1 in accordance with the presentinvention in which 0.02 at % of Co was added to the solder of Run No. 6,surface irregularities were significantly suppressed and no shrinkagecavities were found on the surfaces of the solder balls, and thesurfaces of the solder balls were as smooth as those of solder balls ofa conventional Sn—Pb solder (FIG. 3). The percentages of occurrence ofmounting problems and misidentification in all the solder ballsaccording to the present invention (Runs Nos. 1-5) were as low as thoseobtained with the conventional Sn—Pb solder in Run No. 7.

What is claimed is:
 1. A solder ball made of a lead-free solder whichcomprises, by atomic percent, 3%-6% of Ag, 1%-4% of Cu, 0.01%-2% of atleast one element of the iron group, 0%-4% of P, and a balance of Sn. 2.A solder ball as claimed in claim 1 wherein the content of P is0.04%-4%.
 3. A solder ball made of a lead-free solder which comprises,by atomic percent, 3%-6% of Ag, 1%-4% of Cu, 0.01%-2% of Co, 0%-4% of P,and a balance of Sn.
 4. A solder ball as claimed in claim 3 wherein thecontent of P is 0.04%-4%.
 5. A method of forming solder bumps comprisingplacing solder balls as claimed in claim 1 on a substrate and thenheating the substrate to melt the solder balls and form them into solderbumps secured to the substrate.
 6. A method of forming soldering bumpscomprising placing solder balls as claimed in claim 3 on a substrate andthen heating the substrate to melt the solder balls and form them intosolder bumps secured to the substrate.
 7. A substrate arrangementcomprising a substrate for a BGA package and a plurality of solder bumpsformed on the substrate from solder balls as claimed in claim
 1. 8. Asubstrate arrangement comprising a substrate for a BGA package and aplurality of solder bumps formed on the substrate from solder balls asclaimed in claim
 3. 9. A BGA package comprising a substrate, asemiconductor chip mounted on the substrate, and a plurality of solderbumps formed on the substrate from solder balls as claimed in claim 1.10. A BGA package comprising a substrate, a semiconductor chip mountedon the substrate, and a plurality of solder bumps formed on thesubstrate from solder balls as claimed in claim 3.